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09-24-2010, 12:01 PM   #1
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Very confused about exposure

I am totally new to SLR, trying hard to learn. I have a k-x and these are some of the confusion I am having with exposure and metering

1) I have a manual focus non-A type lens (Phenix 50mm f1.7 to be exact). I followed the instruction here and managed to set it up and I can take pictures with it -- no problem there. Now, if I use M-mode (which is kind of only choice)

a) On a very sunnay day here in Chicago, outside in the Sun I went ahead to take close-up (less that 2 feet) on my flower to test the DOF. I had the lens set up at f1.7. When I pointed to blue sky and pressed the green button, I got a shutter speed of 0.3 sec !!! Can not be right, right ? Sure enough, the photos get burned when at 1/500. So is wrong ? From the book I read, aren't we supposed to get meter reading from blue sky above ?
To make matter much more confusion, if I select f22, I get the same shutter speed, although I can "see" that the lens is being stopped when I pressed the green button.

b) If I get reading off the flower, I get much faster reading, I think 1/3200 or something like that. But even at that, photos are over exposed. In fact, I did not get any decent picture even at the highest speed.
Does this is f1.7 is too much for Sunny day ?

2) Now, when I use the kit lens, I have another different confusion. If I use aperture priority under lower light (basement with overhead halogen type flood light), when I press the shutter halfway, the camera sets the shutter speed (as expected), and I get a reasonable photo. But when I use M mode, which exact same Aperture and shutter speed, under same condition, I get blinking EV warning (> 3.0).
Why is that ? Am I going something wrong ?

09-24-2010, 12:29 PM   #2
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On a bright sunny day F1.7 is way too much light. As you have discovered the K-x defaults to .3 when there is too much light. For a close up photo you will want more depth of field than F1.7 anyway. Try stopping down to F16 (Sunny 16).
09-24-2010, 01:20 PM   #3
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raj, you are indeed doing something wrong.
But that's OK - it's all a learning process.
Firstly, I'd strongly suggest going back to first principles and learning the exposure triad along with the Sunny 16 rule. You will get a good appreciation of what is an approximate correct exposure for ambient light conditions (in bright sun, the settings of Tv 1/125 ISO 100 Av 16 produce the right exposure).

Then learn about when and where to use certain apertures. f/1.7 has such a thin depth of field, it should be limited to few applications such as abstract art and certain portraits for example. Stop down and learn how your camera stops a non-A lens down during exposure (see your manual for permitting the use of a non-A lens and the use of the Green Button).

Not sure about your EV warning in M mode - were your ISO settings kept the same between your Av and M modes? Did you take the photo in M mode despite the EV warning? How did it come out? The same?

Hope this helps. Most of all, enjoy the journey.
09-24-2010, 01:26 PM   #4
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i too am a noobie. I have a pentax M 50mm F1.4 lens and have no trouble shooting on a sunny day wide open. I dont point my camera in the sky before i press the green button. I focus on my target then press the green button and my camera will then set the shutter speed for what is appropiate for that photo.

I tend to avoid shooting wide open at F1.4 because I like to shoot is the absolute sharpest (f2.8-f4) are my favorites. I also tend to be in a lot of low light situations because i work 8am-6pm.

09-24-2010, 01:28 PM   #5
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raj sometimes the iso is set to auto in most modes, when you hit manual the the iso is up to you. if you bump up iso in your basement you will find you will get the correct exposure (using the same settings as auto)

check out the ultimate exposure computer... it is a fantastic resource.
Ultimate Exposure Computer
09-24-2010, 01:33 PM   #6
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Let's go back a few steps,

First of all turn off auto ISO, (it should go off in manual any way) and then set ISO to 200.

then, set metering to matrix (for now) you can learn about spot later. The camera, with a non A lens installed will default to average metering.

Finally, set the aperture to any value you want and press the green button to meter.

the camera should have a histogram centered (roughly)

To really test it, focus on a uniformly lit block wall and then take a picture of the blocks (yeah I know it's boring subject) this should show a vairly narrow spike in the histogram just a little right of center.

Note, there may be some metering errors with the camera and manual aperture lenses. this has been posted many times, but it is useful. it shows the error with my 50mm lens on each camera I have, and each focusing screen I have used in that camera.

You should be pretty close, i.e. within 1-2 stops although some bodies do have a perminant offset, but this is more true with M42 lenses than K mount, unless your lens is painted black or otherwise insulating on the lens mount.


09-25-2010, 09:17 AM   #7
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Folks,

Thank you so much for all the responses.

I think I am getting better, and right off the bat, I now agree that f/1.7 is usually not needed, sure outside. As a corollary question, so where does one use 1.7 or even more prized 1.4 ? Indoors ?

Now, I also discovered, that when using a M type lens (non-A series), there is on extra step needed for K-X. That is, on the menu, we need to program the Green button on Action in M mode to TvShift. The default is program-line, which does screw up things, IMHO.

See official Pentax guide here: http://support.pentaximaging.com/documents/908
09-25-2010, 09:34 AM   #8
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You can use f1.9, f1.8, f1.7, f1.4 and the even more prized, f1.2 outdoors anytime it suits you. You just have to be aware of your meter and what it is telling you.

f1:1.2 (50mm 1/4000) (I've darkened the background on purpose in post processing - before anyone gets the idea this isn't exposed properly)



f1:1.4 (85mm 1/8000)



Granted, in some cases, you're going to need a very fast shutter speed to avoid blowing things out, it's part of the equation. Always has been, always will be. The programming on our cameras (green button) does a nice job for most things but sometimes, it's better to forget it exists, set the mode to M, and go full manual making your own adjustments.



09-25-2010, 09:47 AM   #9
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Glad to hear you are progressing

I have a few additional points. I don't cave a kx myself but on my cameras I find that the green button defaults to Tv shift with a manual aperture lens so I am surprised you need to change this.

As for F1.4 these lenses are sought after for narrow depth of field
09-25-2010, 10:17 PM   #10
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QuoteOriginally posted by boriscleto Quote
On a bright sunny day F1.7 is way too much light. As you have discovered the K-x defaults to .3 when there is too much light. For a close up photo you will want more depth of field than F1.7 anyway. Try stopping down to F16 (Sunny 16).
"Sunny 16" isn't meant to imply that on sunny days one should shoot at f16.
09-25-2010, 10:26 PM   #11
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Sunny 16 rule - Wikipedia, the free encyclopedia

09-25-2010, 10:31 PM   #12
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QuoteOriginally posted by Mike Cash Quote
"Sunny 16" isn't meant to imply that on sunny days one should shoot at f16.
The basic rule is, "On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting]."

Sunny 16 rule
09-25-2010, 10:53 PM   #13
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It is a guideline for estimating exposure when one doesn't have a light meter. It provides a starting point for figuring out shutter speeds.

If you think it means on sunny days you are supposed to shoot at f16 then you are seriously missing the whole point of the thing.

Did you read (and understand) the whole thing? Or did you get to the "basic rule" and just stop reading?

As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1/250 second at f/11 gives equivalent exposure to 1/125 second at f/16.
09-25-2010, 11:03 PM   #14
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As you state, it's a guide and starting point. I'll add that in order to make best use of it, it is helpful to know what constitutes an equal exposure with respect to aperture and shutter speed. It also helps to know where each setting would be most useful.

Sunny (for ISO 100)

f16 at 1/125
f11 at 1/250
f8 at 1/500
f5.6 at 1/1000
f4 at 1/2000
f2.8 at 1/4000
f2 at 1/8000

(f22 at 1/60)
(f32 at 1/30)

With each, it's probably worthwhile to bracket +/- 1 stop or so.

09-27-2010, 06:04 AM   #15
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QuoteOriginally posted by rajchat Quote
Now, I also discovered, that when using a M type lens (non-A series), there is on extra step needed for K-X. That is, on the menu, we need to program the Green button on Action in M mode to TvShift. The default is program-line, which does screw up things, IMHO.
Different people use the Green Button in different ways. I don't use it for TvShift because the eDial will change shutter speed. I also don't use it to set exposure because Av button does that.

Currently I have it set to Optical Preview. That means, when I push it, the aperture gets closed down to the chosen F-stop. The viewfinder gets a bit darker and the background becomes less blurred as a consequence (it's a crude depth-of-field preview). Also, the camera now meters the light and shows the exposure in the viewfinder, as +0.3 or whatever. I can then point the camera at the brightest and darkest parts of the scene and see how over-exposed or underexposed they are. This can be a useful adjunct to using the Av button to set exposure directly.

I find that in manual mode it's easy to forget what settings are at. Especially ISO: if you are getting the 0.3" shutter speed, check the ISO is 200 and hasn't been left at 1600 for some reason. You need to "chimp" more, too: ie after taking a photo, especially the first photo, look down at your camera and check it got exposed right. Either with instant review or play-back mode, and either showing the histogram or with blinking highlights/shadows.
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